According to the recent requirement for automobiles to be fuel-efficient, conjugated diene-based polymers, which have low rolling resistance, high wear resistance, excellent tensile properties, and handling stability represented as wet skid resistance, are required as a rubber material for tires.
To reduce the rolling resistance of tires, there is a method of decreasing the hysteresis loss of vulcanized rubber. As evaluation standards of such vulcanized rubber, repulsive elasticity at 50° C. to 80° C., tan δ, Goodrich heating, and the like are used. That is, rubber materials with high repulsive elasticity at the above-described temperature range or low tan δ and Goodrich heating are preferably used.
As rubber materials having a low hysteresis loss, natural rubber, polyisoprene rubber, polybutadiene rubber, and the like are known, but these materials have low wet skid resistance. Thus, conjugated diene-based polymers or copolymers such as styrene-butadiene rubber (SBR) or butadiene rubber (BR), prepared by emulsion polymerization or solution polymerization, have recently been used as rubber for tires. Among these, the biggest advantage of solution polymerization over emulsion polymerization is that the contents of a vinyl structure and styrene, which determine physical properties of rubber, may be arbitrarily adjusted, and molecular weights, physical properties, and the like may be adjusted by coupling, modification, or the like. Thus, SBR prepared by solution polymerization through which it is easy to structurally change the finally prepared SBR or BR, movement of chain ends may be decreased by binding or modifying the chain ends, and binding strength of SBR with a filler such as silica, carbon black, or the like is increased is widely being used as a rubber material for tires.
When such solution-polymerized SBR is used as a rubber material for tires, a glass transition temperature of rubber may be increased by increasing a vinyl content in the SBR, and thus physical characteristics required for tires, such as driving resistance and braking force, may be adjusted, and fuel consumption may also be reduced by appropriately adjusting the glass transition temperature. The solution-polymerized SBR is prepared using an anionic polymerization initiator, and chain ends of the formed polymer are bound using various modifiers or are modified. For example, U.S. Pat. No. 4,397,994 discloses a technology for binding active anions of chain ends of a polymer, obtained by polymerizing styrene-butadiene in a non-polar solvent using alkyllithium as a mono-functional initiator, using a binder such as a tin compound.
Meanwhile, carbon black, silica, and the like are used as a reinforcing filler of tire treads. When silica is used as a reinforcing filler, low hysteresis loss and wet skid resistance are improved. However, silica with a hydrophilic surface has a lower affinity with rubber than that of carbon black with a hydrophobic surface, and thus has a problem such as poor dispersibility. Thus, it is necessary to use a separate silane coupling agent to enhance the dispersibility of silica and enable binding between silica and rubber. Thus, studies on the introduction of functional groups with affinity or reactivity with silica to an end of rubber molecules have been conducted, but an effect thereof is insufficient.